In radio communication systems, for example, in a WiMAX (Worldwide Interoperability for Microwave Access) system, a MAC (medium access control) layer protocol is a protocol that is common to physical layers. When data is exchanged between a base station and a mobile station, association (e.g., mapping) between MAC layers in the base station and the mobile station is performed to establish a connection.
Data is exchanged using a plurality of packets, which have the format of protocol data units defined by a MAC protocol (the protocol data units may be hereinafter referred to as “MAC-PDUs” or simply “PDUs”). The MAC-PDU has a MAC header field and a payload field. The MAC header field includes information, such as a PDU configuration system, a PDU length, and a connection ID (CID). The payload field includes data. The payload field, however, is optional. As the MAC header, a generic MAC header (GMAC) and a MAC control header are available.
A burst is a group of one or more PDUs and a frame is a group of one or more bursts. A reception device distinguishes a burst field from a frame, checks the connection IDs in the MAC headers of the PDUs in the burst to select PDUs addressed to the reception device, and processes the selected PDUs.
In radio communication, for example, error occurs in reception data, depending on the state of a communication medium between a transmitting end and a receiving end. Thus, in the radio communication, the MAC header field and the payload field have error correction code bits, such as cyclic redundancy check (CRC) bits, in order to allow the receiving end to detect errors. A CRC in the MAC header field is called a header check sequence (HCS) and a CRC for detecting errors in the payload field and the entire PDU is called a frame check sequence (FCS).
A reception processing device in the reception device sequentially performs error checking and data processing on the PDUs in the burst. For example, in a case in which error checking using an HCS bit in the MAC header of the PDU indicates OK, e.g., when the connection ID in the MAC header is addressed to the reception device, the reception processing device performs data processing on the PDU. When the connection ID is not addressed to the reception device, the reception processing device discards the PDU and performs error checking and data processing on a next PDU. The MAC header includes a length field having information indicating the data length of the PDU. The reception processing device extracts a next PDU based on the data length in the length field.
When error checking based on the HCS bit indicates NG (No Good), the data of the PDU is not reliable and thus the front end of a next PDU may not be identified. Thus, processing for discarding the unreliable PDU and all of the subsequent PDU(s) in the burst is performed. With this method, however, all the remaining PDU(s) in the burst are discarded, resulting in a sharp decrease in the throughput.
It has been proposed that, when the error checking based on the HCS bit is NG, the next MAC header is scanned to predict the front end of a next PDU. According to the proposed technology, error checking based on the HCS bit in the MAC header is performed each time the scan is performed through 1-byte shifting, and when the result of the checking is OK, the predicted PDU is determined to be corrected and is thus recovered through data processing. One example is disclosed in Japanese Laid-open Patent Publication No. 2007-195185.
Processing PDUs in a communication system is also proposed in, for example, Japanese Laid-open Patent Publication Nos. 2008-17467 and 2001-358699.
When error checking based on the HCS bit indicates NG, discarding all of the subsequent PDUs in the burst being processed is not preferable since the throughput decreases sharply.
According to the PDU recovery method disclosed in Japanese Laid-open Patent Publication No. 2007-195185, under a communication environment where the amount of noise is large, even when error checking based on the HCS bit in the MAC header indicates OK, it is highly likely that the result of error checking based on a CRC bit in the PDU becomes NG. Thus, even when the PDU recovery processing is performed, the result of error checking based on the CRC bit in the PDU becomes NG and the PDU is eventually discarded. This results in a decline in the performance of the data reception processing.